Snowden condemned the appointment of Condoleezza Rice to the Dropbox board because of her record supporting warrantless wiretapping. Photo: Andy Rain /EPA

Edward Snowden has warned against the cloud storage service Dropbox which he says is "hostile to privacy", and called for more services to offer the 'zero knowledge' which have no decrypted access to user data.

Snowden pointed out that Dropbox had appointed former Secretary of State Condoleezza Rice to its board in April 2014.

"Dropbox is a targeted you know wannabe PRISM partner," he told the Guardian. "They just put … Condoleezza Rice on their board… who is probably the most anti-privacy official you can imagine. She's one of the ones who oversaw Stellar Wind and thought it was a great idea. So they're very hostile to privacy."

Cloud companies need to pursue the zero knowledge system, he said, where they host and process content on behalf of customers without having access to it themselves.

"By depriving themselves of the ability to read the information, of the ability to sort of analyse and manipulate the information without the customers' consent or authorisation, that's the only way they can prove to the customers that they can be trusted with their information," he said.

"Spideroak has structured their system in such a way you can store all of your information on them with the same sort of features that Dropbox does, but they literally had have no access to the content. So while they can be compelled to turn it over, the law enforcement agencies still have to go to a judge and get a warrant to actually get your encryption key from you."

Spideroak, which was founded in Illinois in 2006, is one a group of tech companies offering a zero knowledge service for its customers, who use it to store documents, images and videos.

Much like its larger rival Dropbox, Spideroak offers online storage for individuals and businesses. But because the service encrypts users' files on their device before they reach the Spideroak servers, it claims no one at the company can decode them.

Only the user, with their unique key – an extremely long, unguessable number – can decrypt the files again. This technique means that data can't be at risk from internal or external hackers, and any government would have to request access by issuing a warrant to the user for their password.